1. Field of the Invention
This invention relates to a recording head which performs recording through utilization of heat energy.
2. Description of the Prior Art
Recording methods for performing recording by utilization of heat energy have heretofore been attracting attention for generating very low noise during recording because of non-impact and also for their application to color printing which has been developed in recent years. The recording head to be used in a thermal printer having such a thermal recording method has generally a constitution comprising a glaze layer having smoothness, which is an electrical insulator and also functions as the upper layer for controlling the accumulation of the heat generated, provided on a substrate of a good thermal conductor such as alumina ceramics, a heat-generating resistor on said substrate and a pair of electrodes connected electrically to the heat-generating resistor, in at least a part thereof. According to information to be recorded, electrical signals are inputted into the above heat-generating resistor, whereby heat energy is generated from the heat-generating resistor and recording is effected by utilization of this heat energy.
In a recording head for performing recording by use of heat energy as employed in a thermal printer, as the lower layer provided on the substrate, glass, quartz, etc. comprising SiO.sub.2 as the main component has been employed in the prior art.
Also, other than the thermal printer as mentioned above, the ink jet recording method (liquid jet recording method) known as a non-impact system recording method is also recently attracting attention in that generation of noise during recording is negligibly small, that high speed recording is possible and also that recording can be effected on the so called plain paper without special treatment of fixing.
Among them, for example, liquid jet recording methods as disclosed in Japanese Laid-open Patent Publication No. 51837/1979 and German OLS No. 2843064 have a different specific feature from those of other liquid jet recording methods in that driving force for discharging droplets is obtained by permitting the heat energy to act on liquid.
More particularly, the liquid jet recording method disclosed in German OLS No. 2843064 is not only applicable for the so called drop-on demand recording method, but also has the specific feature of being capable of providing images of high resolution and high quality at high speed, because the recording head portion can easily be embodied into a recording head of the full line type and high density multi-orifice.
The recording head portion in the device to be applied for the above liquid jet recording method is provided with an orifice for discharging liquid, a liquid discharging portion connected to said orifice having a heat-acting portion which is the portion where heat energy acts on the liquid for discharging droplets and an electrothermal transducer as a means for generating heat energy.
The electrothermal transducer is provided with a pair of electrodes and a heat-generating resistance layer connected to these electrodes having a region for heat generation (heat-generating portion) between these electrodes.
A typical example showing the structure of such a recording head to be used for the liquid jet recording method is shown in FIG. 1A and FIG. 1B.
FIG. 1A is a partial front view as viewed from the orifice side of a recording head to be used for the liquid jet recording method according to the present invention, and FIG. 1B is as partial sectional view taken along the dot and dash line XY as shown in FIG. 1A.
The recording head 101 shown in the Figures has a structure having an orifice 105 and a liquid discharging portion 106 formed by bonding a grooved plate 104 provided with a desired number of grooves with a certain width and a depth at predetermined line density to the substrate 103 on which an electrothermal transducer 102 is provided so as to cover over the surface of said substrate. In the case of the recording head shown in the Figures, it is shown to have a plurality of orifices 105. However, the present invention is not, of course, limited to such an embodiment, but a recording head in the case of a single orifice is also included within the scope of the present invention. The liquid discharging portion 106 has an orifice 105 for discharging liquid at its terminal end and a heat-acting portion 107 which is the site where the heat energy generated from the electrothermal transducer acts on liquid to generate bubbles, thereby causing abrupt changes in state through expansion and shrinkage of its volume.
The heat-acting portion 107 is positioned above the heat-generating portion 108 of the electrothermal transducer 102, with the heat-acting face 109 as the face which comes into contact with liquid being the bottom face.
The heat-generating portion 108 is constituted of a lower layer 110 provided on the substrate 103, a heat-generating resistance layer 111 provided on said lower layer and an upper layer 112 provided on said heat-generating layer 111. The heat-generating layer 111 is provided on its surface with electrodes 113 and 114 for passing current through said layer 111 for geneartion of heat. The electrode 113 is the electrode common to the heat-genearting portions of respective liquid discharging portions, and the electrode 114 is a selection electrode for generating heat by selecting the heat-generating portion of the each liquid discharging portion and provided along the liquid channel of the liquid discharging portion.
The upper layer 112 serves to protect the heat-generating resistance layer 111, that is, for protecting chemically and physically the heat-generating resistance layer in the heat-generating portion from the liquid employed the upper layer 112 separates the heat-generating resistance layer 111 from the liquid filling the channels in the liquid discharging portion 106 and also prevents the electrodes 113 and 114 from short circuit through the liquid.
The upper layer 112 also serves to prevent electrical leak between adjacent electrodes. Particularly, it prevents electrical leak between the respective selection electrodes or it prevents electrical corrosion caused by the contact between the electrode beneath each liquid channel with liquid which may occur for some reason and current passage through such contact is important. For this purpose, the upper layer 112 having such a function of protective layer is provided at least on the electrode beneath the liquid channel.
Further, the liquid channel provided at each liquid discharging portion is connected through the common liquid chamber constituting a part of the liquid channel upstream of each liquid discharging portion, and the electrodes connected to the electrothermal transducer provided at each liquid discharging portion are provided, for the convenience of designing thereof, so as to pass below said common liquid chamber on the upstream side of the heat-acting portion.
Accordingly, also at this portion, the above-mentioned upper layer is generally provided for the purpose of preventing contact between the electrodes and the liquid.
Whereas, as the characteristics required for the lower layer of a thermal head or the lower layer of a liquid jet recording head, the following characteristics are primarily important:
a. to have good heat resistance which can stand the heat generated at the heat-generating portion of the heat-generating resistance layer;
b. to have good thermal impact resistance which can stand repeated heat generation at the heat-generating portion of the heat-generating resistance layer;
c. to have a coefficient of thermal expansion substantially equal to that of the heat-generating resistance layer and that of the electrode layer laminated on the lower layer;
d. to have good adhesion to the respective layers laminated on the lower layer.
When these characteristics are fully satisfied, the recording head has a long life and high reliability. In addition, from the viewpoint of preparation of the recording head, in formation of the heat-generating resistance layer to a desired shape which is generally done according to the photolithographic step, if the etching speed ratio of the lower layer to the heat-generating resistance layer is not sufficiently great, there is also involved the problem such that an unnecessary portion of the lower layer may be etched or side etching may occur to lower the life of the completed head. Thus, the lower layer is required to have great etching resistance as one of the important charactcristics.
Another important role of the lower layer is control of the heat generated from the heat-generating resistance layer. During recording, it is required to transmit necessary and sufficient heat toward the liquid side and also to permit unnecessary heat to be dissipated rapidly toward the substrate side. If this control of heat cannot be done well, there may be caused bad influences such as worsening of response to input of electrical signals to the electrothermal transducer or destruction of members constituting the recording head such as the electrothermal transducer, etc. through accumulation of heat. Particularly, in recent years, a recording head with high response characteristic is highly desired, because tone recording characteristic and high speed recording performance are demanded. For satisfying such requirements, the substrate constituting the recording head is desired to be made of a material having excellent heat dissipating characteristic and heat accumulating characteristic. Further, for permitting a substrate having such characteristics to function fully effectively, the lower layer is required to be formed of a material having high thermal conductivity.
However, no lower layer which can satisfy all of the requirements as mentioned above has been proposed yet. For example, in the case of a glaze layer preferably employed for a thermal head, since the heat resistance is to a temperature of about 500.degree. C. to 800.degree. C., the temperature which can be reached by the heat energy generated by the heat-generating resistance layer will be suppressed in the vicinity of such a temperature. In the case of performing recording at temperatures higher than the above temperature, it has been required to provide a layer having high heat resistance on the lower layer or devise the method for driving the electrothermal transducer.
On the other hand, in aspect of etching resistance, since the lower layer has an etching resistance on the same level as or lower than that of the heat-generating resistance layer, it may sometimes lower the yield in etching process or the reliability of the recording head. For this reason, side etch has been prevented in the prior art by a contrivance such as providing further an etching resistant layer such as of Ta.sub.2 O.sub.5, etc. excellent in etching resistance on the glaze layer, thereby preventing lowering in reliability of the recording head. In addition, also in aspect of thermal impact resistance, the glaze layer composed mainly of glass involves the problem of generation of cracks, etc., and also has the problem of very poor adhesion to the heat-generating resistance layer and the electrode layer because of the coefficient of thermal expansion which is greatly different from that of each of such layers (composed mainly of metals).
Also in aspect of thermal conductivity, there has been involved the problem that the temperature of The glaze layer itself of the prior art is elevated during high speed recording, whereby response characteristic of the recording head was worsened to worsen the quality of recording.
Further, in the heat-oxidized SiO.sub.2 of Si which has been known to be preferably used for the lower layer in the liquid jet recording head, etching resistance, particularly thermal expansion ratio and thermal conductivity as the lower layer for the liquid jet recording head also suitable for high speed recording could not fully be satisfactory in some cases. Thus, no bubble jet recording head excellent in overall use durability when performing high speed recording continuously for a long time has been provided.